In medicine, sterilization of instruments is known to be commonly required before they can be safely allowed to come into contact with patients. The same is true of other tools and even drugs and linens. Sterilization is also required in other fields such as biology, pharmacology, and veterinary medicine. It is typically performed in a pressurized chamber such as in an autoclave by producing steam from distilled or mineral-free water and heating it. In a typical sterilization cycle, steam temperatures of 137.degree. C. and pressures of up to 3 bars are achieved and maintained for a predetermined number of minutes.
Exemplary of current art are autoclaves manufactured by Tutnauer U.S.A. Co. Ltd., which use electric heaters mounted on the external surface of the autoclave cylindrical chamber using bolted connections. The heating elements in the heaters are nickel-chromium wire which are mounted on a flat micanite framework with two micanite insulating liners enclosed in an aluminum cladding which is in thermal contact with the external surface of the autoclave cylindrical chamber.
In a typical sterilization cycle, electrical current is passed through the nickel-chromium wire which produce heat which is transferred through the micanite liners to the aluminum cladding and therefrom to the external surface of the autoclave chamber. The chamber, which has been sealed with the items to be sterilized an a necessary quality of distilled or mineral-free water, heats until the water is evaporated to steam and the required temperature and pressure are achieved. When the sterilization cycle is completed, clean, dry air is circulated in the chamber to cool and dry the chamber and its contents.
These heaters have a number of disadvantages. They are inefficient because of the high thermal resistance of the micanite, the large contact surfaces between the nickel-chromium wire and the micanite liners, between the liners and the aluminum cladding, and between the cladding and the autoclave chamber surface, and because only one side of the heater contacts the chamber for heat transfer thereto. A further problem with single-sided heat transfer is the risk of overheating on the other side since there is no direct removal of heat therefrom. Overheating of the entire heater unit is also possible in the absence of sufficient water in the chamber. This requires temperature limiters, such as thermostats and power controllers which make the entire autoclave more complicated, more expensive, and less reliable.
U.S. Pat. No. 5,520,892 discloses sterilization unit for dental and other instruments which employs thermistors with positive temperature coefficient of resistance (PTC) as heating elements. The are self regulating with respect to temperature, thereby eliminating the above-mentioned overheating problems. The unit disclosed employs two-stage heating to provide very precise temperature regulation and has other features required in certain specific applications. It also has a relatively small chamber for objects to be sterilized.
U.S. Pat. Nos. 4,677,280 and 4,529,868 also disclose sterilization units employing PTC heating elements, but both of these are very specifically applied to sterilization of contact lenses.
None of the above devices are applicable to the full range of sizes and types of sterilization units and none can be fitted as heaters on existing autoclave units.